In the past motorcycle front forks have been constructed using telescopic fork tubes running from a pair of triple clamps to the front wheel axle on either side of the front wheel. Most front forks have been constructed using the outer of the two tubes mounted in the lower position. This construction gained great popularity due to its compatiblity with a simple damping system and low cost of manufacture. As time went on there was a search for a construction allowing more travel and better rigidity. A significant improvement came when nearly all manufacturers went to a design with the axle mounted forward of the larger cylinder. This construction permitted maximum travel and overlap with a minimum distance between the axle and the top of the fork tube. The additional travel reduced the rigidity and therefore the precision of the steering characteristics of the motorcycle. Also the bottom of the fork hit the ground often if the motorcycle was ridden in ruts or around the right type of corner.
The object of the present invention is to provide a lightweight, highly rigid fork with low friction, high quality damping characteristics and no axle overhang.
One feature of the present invention is the provision of a telescopic design with the large diameter tube mounted in the triple clamps and the small diameter tube attached to the axle wherein the inner tube is allowed to slide up through the outer tube until it hits the fork cap at the top of the outer tube.
In accordance with this object of the present invention mounting of the larger diameter tube in the triple clamp produces a higher strength to weight ratio and therefore a greater possibility of higher rigidity. Also the length of the upper tube that is clamped between the triple clamps can be utilized for travel of the telescoping lower tube.
In accordance with another feature of the present invention an upper bushing is provided in the upper tube held by bushing retainers and reduces distortion that would prevent the lower inner fork tube from sliding freely through the upper outer fork tube.
In accordance with another feature and advantage of the present invention the front axle is placed in front of the center line of the fork to reduce the moment of inertia of the fork assembly.
In accordance with still another aspect of the present invention a small outside diameter, primary extension spring is housd in a spring cylinder which retains the spring perfectly straight at all times and thereby avoiding snaking of the spring and maintaining the weight of the fork member low because of the smaller diameter spring.
In accordance with still another aspect of the present invention a bottoming piston is provided which has a precise outside diameter which operates in conjunction with a hollow cylindrical damper bushing which slides on a damper rod and with an enlarged diameter opening at the upper end of the bushing with a very precise inside diameter. In accordance with this aspect of the present invention when the fork assembly is under compression and extension a precise clearance between the rod and the bushing allows a small amount of oil to fill the enlarged bottom out chamber of the damper bushing housing. When the system is just short of full travel the bottoming piston starts into the bottom out chamber and displaces oil which is forced through the clearance between the damper rod and bushing and between the bottom out piston and the damper bushing creating a heavy damping effect thereby cushioning any metal-to-metal bottoming.
In accordance with another apsect of the present invention, the fork member includes at least one, and preferably a pair, of spaced-apart bearing housings secured to the outer fork tube with the outer fork tube opening into said housing on opposite sides of said outer fork tube. A pair of rollermembers is mounted in the housing on opposite sides of the outer fork tube and extending through the opening in the outer fork tube, each of said roller members having a concave surface with the axis of said concave surface coincident with the axis of said roller member, and the concave surface engaging the exterior surface of the inner fork tube.
These features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in conjuction with accompanying drawings wherein similar characters of reference refer to similar parts in each of the several views.